Questions have been raised about the UK's exit strategy in Afghanistan after three British soldiers were killed and four injured by an Afghan soldier in Nahr-e Saraj in Helmand province on Tuesday.

The men, from 1st Battalion The Royal Gurkha Rifles, were on duty at a checkpoint when the Afghan opened fire.

Lt Col James Carr-Smith, Task Force Helmand

"We believe these were the actions of a lone individual who has betrayed his Isaf and Afghan comrades.

"His whereabouts are currently unknown but we are making strenuous efforts to find him. He should know that his actions will not deter us from our task and we will continue to work closely with our Afghan friends to bring security to Helmand.

"Three courageous and dedicated soldiers have made the ultimate sacrifice. They will be greatly missed and their actions will not be forgotten. We will remember them."

Prime Minister David Cameron

The prime minister said: "Our hearts go out to the brave Gurkha soldiers and their families. This is an appalling incident. I think it needs an immediate and urgent investigation, and I have discussed that with President Karzai this morning.

But I think it is absolutely essential that we don't let this terrible incident change our strategy, or change our approach.

It is the right thing to do to build up the Afghan National Army. There are 120,000 of them now and they are doing a good job, working with Nato allies.

"We must not let this change our strategy. We need to build up that army because that is the way that we will be able to bring our troops back home in the end."

He added that what had happened was because of a "rogue element" in the Afghan National Army and it was not the time to lose faith in the strategy as that was what the insurgents would want.

"In the end, it is when the Afghans can take care of their own security, can keep al-Qaeda out of their country, can make sure there is basic stability and security, when that happens we will be able to bring our troops back home."

Secretary of State for Defence Liam Fox

"This is a despicable and cowardly act, and my thoughts and prayers go out to the family and friends of those who have lost their lives.

"This incident will be thoroughly investigated by Isaf and by the Afghan security forces, and we will do everything we can to bring the individual responsible to justice.

"Training and development of the Afghan National Security Forces is vital to the international security mission in Afghanistan and today's events will not undermine the real progress we continue to make. British and Isaf forces are working shoulder to shoulder with Afghans and will continue to do so undeterred."

Gen David Petraeus, Nato commander in Afghanistan

"I echo the condolences and sentiments offered by President Karzai and the other Afghan officials.

"This is a combined, joint mission - Afghan and alliance troopers fighting shoulder to shoulder against the Taliban and other extremists.

"We have sacrificed greatly together, and we must ensure that the trust between our forces remains solid in order to defeat our common enemies.

"On behalf of all the troopers of Isaf, I offer sincere condolences to the families and the fellow service members of our fallen comrades."

Deputy Commander of Isaf Forces, Lt Gen Nick Parker

He told Nato: "In that patrol base, this will be a traumatic event and what I say to them is, 'Keep on working, you're doing a fantastic job and you must continue to do it and the vast majority of your Afghan partners are real genuine partners and you'll know that because you work together every day'.

"But more widely across Helmand I think everybody knows partnering is the key to our future.

"We have got to transfer security responsibility to the people whose country this is and if we don't do that, we're not going to succeed in our mission."

Afghanistan government

President Karzai's spokesman Waheed Omar said the Afghanistan government was also investigating the killings.

He said: "All I can say at the moment is that we learnt, with regret, that this incident has happened.

"We don't have more details of this and the president was saddened to hear this.

"He has asked the ministry of defence to come up with more information as to the nature of this attack, how it happened, what are the causes and obviously who was killed and injured.

"And if it is confirmed then it is a very unfortunate attack and the government of Afghanistan will do everything to make sure the perpetrators are brought to justice

Afghan Army Chief of Staff Gen Sher Mohammad Karimi

"The loss of any of our coalition partners affects us deeply.

"We extend our thoughts and prayers to the friends and families of our fellow soldiers.

"Our ongoing, partnered investigation will seek to determine how this event could have occurred and we will prosecute those responsible."

Col Richard Kemp, ex-commander of UK troops in Afghanistan

Col Kemp told the BBC it was important to remember that this was an isolated incident and "not a pattern of events".

He said: "It will make our soldiers more cautious and a maybe bit more jumpy when dealing with Afghans inside their own bases with Afghan troops."

He added that measures could be taken to safeguard against such incidents in the future but they would affect relations with the Afghan troops and would not be "desirable".

He said: "The sort of thing you could do is to not allow Afghan forces to be armed with live ammunition in the same base as British troops unless they're about to go out on patrol, but that would severely undermine trust.

"You could segregate bases between the two - but that would not only undermine trust, but it would also prevent the essential partnering that's taking place."

Maj Gen Patrick Cordingley, former army commander

A former commander in the first Gulf War, he told the BBC it was important to find out exactly what had happened as "feelings are going to be running high not only among the soldiers involved but also in this country where people are going to continue to ask the question 'Is it all worthwhile'?"

He added: "The British soldiers and marines who have been working with the Afghan soldiers and police have been extremely good at doing it and have to come to respect a lot of the people they have been working with. Now, clearly that's not the case now.

"It is this whole business now of trying to overcome the problem and not let feelings run high and this business of trying to work out if there are some rogue elements in those soldiers and policemen who they're trying to train, but that's going to be difficult if you don't speak the same language."

Capt Doug Beattie, Afghan war veteran and author

Capt Beattie served two tours in Helmand with the Royal Irish Regiment and worked closely with the Afghan National Army.

He said the deaths should not distract from the progress that was being made.

"This sort of thing is thankfully rare, and this really should be seen in isolation.

"Since 2006 we have trained 130,000 Afghan soldiers and right now 5,000 of the 10,000 British soldiers in Helmand are actually partnering the Afghan army.

"So it must be viewed in isolation, and it's only happened three times since 2006.

"I don't think this is going to derail us in any shape or form.

"I think the partnering of the Afghan army has to be the way forward, and I think we will continue to do that."